Mutations at the diabetes (db) locus on Chromosome 4 of the mouse produce obesity/insulin resistance syndromes. Induction of frank diabetes requires not only the expression of the db gene, but also a destructive interaction between the obesity gene and other modifying genes in the inbred background genome. The objectives of this research are to define the molecular basis of the db mutation at the gene and protein levels, and to define those background modifier genes interacting with the db mutation to effect either unrestricted hyperplasia of the pancreatic beta cells (in diabetes resistant inbred stains) or beta cell necrosis and development of frank diabetes in susceptible inbred strains. Diabetogenesis in the db model is steroid-dependent, with androgens exacerbating and estrogens protecting against development of hyperglycemic stress against pancreatic beta cells. The nature of sex steroid control of diabetogenesis will be analyzed. We have demonstrated that a male-specific glucose intolerance syndrome in a C3H.SW/Lt stock, characterized by hyperinsulinemia, transient hyperglycemia, and hy-perplasia of the pancreatic beta cells in the absence of obesity is associated with a Y chromosome-linked locus controlling steroid sulfatase (STS) activity. Our experiments will test whether genes controlling differential levels of sex steroid sulfurylation/desulfurylation are the key background modifiers of diabetogenic sensitivity. Fibroblast cultures from postnatal mice will be utilized to analyze the primary defect produced by mutation at the db locus at the cellular level. The question of whether insulin receptors from db/db fibroblasts undergo aberrant post-translational modifications will be studied. Finally, the association of the db3J allele with a unique 8.5 kilobase restriction fragment when genomic DNA is hybridized with a probe recognizing a retrovirus- related sequence will be pursued to assess whether db3J is the consequence of insertional mutagenesis. Efforts will be directed to subcloning flanking genomic segments derived from this unique restriction fragment in order to produce a specific probe for the db3J locus.